FIG. 1 and FIG. 2 are circuit diagrams of a capacitive fingerprint sensor using a thin film transistor according to a conventional art.
As illustrated in FIG. 1 and FIG. 2, in the capacitive fingerprint sensor using the thin film transistor according to the conventional art, since a source follower is included within a pixel by using an active pixel structure, an analog-to-digital converter (ADC) recognizes a variation in capacitance depending on a fingerprint as a variation in voltage by data read-out, thereby enabling the sensor to drive without a preamplifier. A reset may be performed using a VDD voltage in a pixel.
The conventional capacitive fingerprint sensor uses a scanning signal or a method in which different voltages from each other are applied to a gate of T2 during one frame at a time depending on the fingerprint by applying a pulse voltage Vpulse which enables separate capacitive coupling to be performed, thereby enabling different currents from each other to flow. At this time, when a capacitance by a fingerprint is Cfp, and the pulse voltage is ΔVpulse, the voltage applied to the gate may be represented by Math Formula 1 as follow.
                              Δ          ⁢                                          ⁢                      V                          g              ⁢                              -                            ⁢              T              ⁢                                                          ⁢              2                                      =                                                            C                s                            +                              C                                  para                  ⁢                                      -                                    ⁢                  T                  ⁢                                                                          ⁢                  1                                                                                    C                s                            +                              C                                  para                  ⁢                                      -                                    ⁢                  T                  ⁢                                                                          ⁢                  1                                            +                              C                                  para                  ⁢                                      -                                    ⁢                  T                  ⁢                                                                          ⁢                  2                                            +                              C                                  f                  ⁢                                                                          ⁢                  p                                                              ×          Δ          ⁢                                          ⁢                      V            pulse                                              [                  Math          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]            
Here, ΔVg-T2 is the voltage applied to the gate of T2, and a value thereof is changed depending on the capacitance Cfp generated by the fingerprint.
As reviewed above, in the fingerprint sensor according to the conventional art, only the capacitive method using the capacitance generated by the fingerprint is mainly used, but the configuration of the fingerprint sensor capable of sensing the fingerprint using various methods is not provided.